1. Field of the Invention
The present invention relates to methods and devices intended to provide assistance to the driving of motor vehicles, particularly in daytime driving, when the external brightness is variable to a considerable degree, for example in sunny weather, when various events may alternatively illuminate the road scene strongly or darken it.
2. Description of the Related Art
Various solutions have been tested for the mitigation of this situation.
In a conventional solution, the driver wears sunglasses in order to avoid being dazzled by the sun and to distinguish the details of the road scene in front of the vehicle more clearly.
The drawback of these sunglasses is that the driver must remove them whenever the vehicle travels through a shaded area or a tunnel, or when the sun is obscured by dense clouds. This is because the attenuation provided by sunglasses, which is intrinsically constant, has the same effects in the absence of any inconvenience due to the sun, making it harder for the driver to perceive the details of the road scene.
An improvement of these conventional sunglasses has been the provision of photochromic glasses lenses which are sensitive to the ambient light level. These lenses have the property of changing color according to the amount of ultraviolet radiation to which they are subjected. The well-known drawback of these photochromic lenses is that they only return very gradually to their state of greater clarity in the absence of ultraviolet radiation, the time taken for the return to the clear state being much longer than the time taken for coloration.
A further drawback is that, since they only react to the presence of ultraviolet rays, their use for driving motor vehicles generally yields no benefit, as the windshields of most modern vehicles block the transmission of these ultraviolet rays. Photochromic lenses are therefore ineffective for preventing the dazzling of drivers of motor vehicles due to large variations of external brightness.
Other solutions, more complicated than simple sunglasses or photochromic sunglasses, have been proposed. For example, U.S. Pat. No. 3,961,181 discloses a screen for vehicle drivers, which protects both eyes separately but simultaneously from the effect of daytime dazzling due to sunlight and nighttime dazzling due to headlights of approaching vehicles. The screen includes a series of vertical, juxtaposed, contiguous cells which can be controlled electrically, for example liquid crystals, these cells being transparent in the absence of an applied voltage, and darkening as a function of the voltage applied. An array of photosensitive sensors whose electrical resistance increases with brightness is associated with the series of cells. Shields are placed in front of the sensors to create shadows on these sensors. Electrical control means are interposed between the sensors and the cells of the screen to control the transmission factor of the cells as a function of the signals received from the various sensors.
This structure is relatively difficult to create and adjust, the aim being to darken only those cells located between the source of dazzling and the driver's eyes.
Additionally, U.S. Pat. No. 4,848,890 discloses glasses whose lenses are formed by a matrix of liquid crystal cells, and whose frame is provided with a directional photosensitive sensor. Depending on the direction from which the solar rays arrive, cells are switched to an opaque state to prevent the wearer of the glasses from being dazzled by the sun. The major drawback of this arrangement relates to the fact that a large part of the glasses lenses, practically a quarter of the surface of each lens, is darkened, causing a reduction in the field of view that is incompatible with driving a motor vehicle.
EP 0 498 143 which is equivalent to U.S. Pat. No. 5,258,607, also discloses also discloses an active anti-dazzle screen for motor vehicle drivers. This screen, which is fixed to the windshield of the vehicle or may take the form of glasses lenses, includes an ambient brightness sensor, while a measurement circuit which also compares the measured value with a threshold value controls the state of transparency of the liquid crystals forming the glasses lenses. The lenses are totally transparent in the absence of a measurement signal. The drawback of this arrangement is that it operates in binary mode, in an on-off way, the lenses being in a state of maximum or minimum transparency depending on whether the brightness is below or above a predetermined threshold.
Additionally, U.S. Pat. No. 4,286,308 discloses an anti-dazzle device for nighttime driving, comprising a variable-transmission screen located in front of the driver, headlights capable of being switched rapidly from an illuminated state to an extinguished state, a sensor of the brightness of the road scene in front of the vehicle, and a control unit. When the exterior brightness level is below a predetermined limit, the variable-transmission screen is in its state of maximum transmission.
On the approach of vehicles traveling in the opposite direction, the control unit automatically controls the transmission/absorption ratio of the variable-transmission screen, in accordance with the brightness detected by the sensor, in order to increase the apparent density of the screen. If the exterior brightness exceeds a predetermined value, the duration of the periods for which the variable-transmission screen is opaque is at a maximum, so that the screen is transmissive only when the headlights are illuminated, thus providing maximum reduction of dazzling.
Document WO 9620846 discloses a method and a device for attenuating the light from the headlights of approaching vehicles, comprising the emission of light pulses by the vehicle headlights and the control of the transparency of filters in front of the driver's eyes in synchronization with the light pulses emitted by the headlights of the vehicle, the transparency of the filters being at a maximum for a duration exceeding that of the light pulses. In this way, the brightness of oncoming headlights is reduced.
The last two of these documents describe systems which are cumbersome and difficult to use, which are relatively slow in operation with relatively long response times, and in which the transparency of the filters or screens used is always less than 50%; that is to say these systems, using liquid crystals, cause a decrease in the perceived luminous intensity, even when their transmission is maximal.
What is needed, therefore, is a method and device that overcomes one or more of the problems in the prior art.